- check_imap_receive is a highly flexible module using an IMAP search
to check an INBOX or folder for messages matching certain criteria.
Warnings and critical alerts can be given based on minimum or maximum
numbers of matching messages. SSL support is available.

- check_email_delivery uses the other two plugins (or you may provide
alternatives) to check full email sending/delivery loops. As well as
alerting via nagios, running statistics can be recorded to a file.